1. Field of the Invention
This invention relates to the strengthening of glass by forming a ceramic coating on the glass surface. More particularly, this invention relates to a process for strengthening glass by coating it with a preceramic layer and then converting the preceramic layer to a ceramic coating.
2. Description of the Related Art
Microcracks on the surface of glass articles are the main reason for the catastropic failure of glass under considerably low tensile load. Therefore, in the manufacture of glass materials such as, for example, glass sheets or shaped objects such as a glass bottle, it is known to protect the glass surface from abrasion and formation of microcracks by coating it with organic polymer materials. For example, Yoshida et al U.S. Pat. No. 4,370,385 describes coating a glass vessel with an organopolysiloxane composition to provide a scuff-masking coating over the glass.
Wieczorrek et al U.S. Pat. No. 4,409,266 provides a shatterproof coating on glass by applying to the glass surface a silane adhesion promoter and a two component system which reacts to form a polyurethane binder.
Kurita et al U.S. Pat. No. 4,656,221 conceals graze marks on a glass bottle by coating the glass with a composition formed from polydiorganosiloxane components and a surfactant. The coating is applied to the glass surface as an emulsion and allowed to air dry.
However, the problem with the use of organic coatings on glass is that the coatings are not very hard, suffer from weathering degradation, and do not increase the actual strength of the glass.
In general, the formation of ceramic coatings on substrates through the pyrolysis of preceramic polymer coatings is known. For example, Gaul, in U.S. Pat. Nos. 4,395,460 and 4,404,153 forms a silicon carbide coating on a substrate by coating the substrate with a polysilazane polymer and then heating the coated substrate in an inert atmosphere or in a vacuum to an elevated temperature of at least 750.degree. C.
Seyferth et al U.S. Pat. Nos. 4,482,669; 4,720,532; 4,705,837; and 4,645,807 teach forming an oxidation-resistant coating on otherwise oxidizable materials such as pyrolytic graphite by application of a preceramic polymer coating over the materials followed by pyrolysis of the preceramic coating to form a ceramic coating. The preceramic polymer materials respectively used in the Seyferth et al patents include (1) polysilazanes that are synthesized by strong base catalyzed polymerization of cyclomethylsilizane, which is the ammonolysis product formed by reacting anhydrous ammonia with a mixture of dihalohydridesilanes and trihalosilanes; (2) polymers formed by reacting polysiloxane with a polysilylamide, which is an intermediate potassium salt of the polymer in (1); and (3) polymers formed by reacting a organopolysilane of the formula [(RSiH).sub.X (RSi).sub.Y ].sub.N with alkali metal amides or silylamides.
Coblenz et al, in an article entitled "Formation of Ceramic Composites and Coatings Utilizing Polymer Pyrolysis", on pp 271-285 of a publication entitled "Emergent Process Methods For High-Technology Ceramics", edited by Davis et al and published in 1984 by Plenum Publishing Corporation, describe the coating of carbon and silicon nitride materials with a dimethylsiloxydiphenylsiloxycarborane polymer and with a silazane oligomer. They report that the resulting coatings were of poor quality with shrinking and cracking of the coatings noted.
Winter et al U.S. Pat. No. 3,892,583 and Verbeck U.S. Pat. No. 3,853,567 describe the formation of shaped articles of homogeneous mixtures of silicon carbide and silicon nitride. The homogeneous mixtures are also said to be useful in forming films, flakes, and coatings.
Baney et al U.S. Pat. No. 4,666,872 describes coating substrates with R.sub.3 SiNH-containing silazane polymers to which has been added certain precious metals, followed by heating the substrate to an elevated temperature of at least 750.degree. C. in an inert atmosphere or vacuum to form a ceramic coated article.
Bujalski U.S. Pat. No. 4,668,642 discloses coating substrates with R.sub.3 SiNH-containing silazane polymers to which has been added certain boron compounds, followed by heating the substrate to an elevated temperature of at least 750.degree. C. in an inert atmosphere or vacuum to form a ceramic coated article.
While the use of such ceramic materials may generally protect the surface of a substrate against scratching or other abrasive action, as well as imparting some oxidation protection for oxidizable substrates such as the above described carbonaceous materials, such coatings are not generally known to impart any physical strength to the substrate itself when applied as thin layers, e.g., less than 1 millimeter in thickness.
Glass materials, however, in addition to needing surface protection such as afforded by the above discussed organic coating materials, need physical strengthening as well. For example, despite the theoretical high strength of silicon-based glass, e.g., in the range of 2000 kpsi, the actual strength of a manufactured glass article may be lower by more than two orders of magnitude, e.g., to a level of 5-20 kpsi.
This dramatic reduction in rupture strength may be explained by the presence of "Griffith flaws", which probably exist on the molecular level, and which serve as weak points to initiate crack propagation and lead to glass failure under stress. Of course, large visible cracks formed by abrasion forces further degrade the rupture strength. However, this latter problem is solved, at least in part, by the aforementioned organic coatings applied over the glass surface.
Stresses and strains formed during the arrangement of the silicate polymers when molten glass is shaped are also responsible for a decrease of strength. The stresses can be relieved by an annealing process which increase the glass strength by a factor of 2. In contrast, however, inducing strains by a tempering process may provide glasses that withstand tensile loads of 40-60 kpsi. However, such tempering processes are not always practical in some forms of glass manufacture, such as, for example, glass container or glass fiber manufacturing.
It would, therefore, be advantageous to provide an economical means for increasing the rupture strength of glass materials. Quite surprisingly, we have discovered a process for treating a glass surface to impart increased rupture strength to the glass body.